This invention relates to a photoconductive iodinated poly(vinyl carbazole) composition and its method of preparation. More particularly, the present invention relates to a photoconductive iodinated poly(vinyl carbazole) composition formed by reacting a source of iodine with a poly(vinyl carbazole) prepared by precipitation polymerization.
Photoconductive polymers are useful in electrostatographic processes wherein an electrostatic latent image is produced on a surface of the polymer. The electrostatic latent image then is developed with a colored toner composition by means well-known in the art. For use in electrostatographic processes, a layer of the photoconductive polymer can be applied to a base sheet such as paper or a transparent or opaque polymeric composition or can be applied to a conductive plate. In the former instance, the latent image is generally developed and subsequently is fused thereon. In the latter instance, the latent image generally is developed and fused thereon or transferred to a base sheet such as paper and is fused thereon.
The most commonly employed photoconductive material wherein development and fusion is effected on the same surface comprises a composition of photoconductive zinc oxide particles dispersed in a transparent resin binder which composition is coated on a paper sheet. However, these compositions are opaque and are not useful in forming a developed image on a transparent background such as microfilm or transparencies. In addition, images developed on zinc oxide are undesirably grainy. In order to permit forming transparencies, the photoconductive layer and any substrate to which it is applied must be transparent. Such photoconductive compositions should have a high dark resistivity; that is, should be capable of holding a high proportion of electrostatic charge applied thereto in the dark. In addition, the photoconductive composition should be capable of quickly dissipating the applied electrostatic charge when exposed to light. Furthermore, the photoconductive composition should be capable of being sensitized so that it is spectrally responsive in the visible region of the spectrum without the need for employing high concentrations of sensitizing dyes that would significantly reduce the transparency of the composition.
It is known that iodinated poly(vinyl carbazoles) exhibit photoconductive properties. However, either for reasons associated with the known methods for preparing these polymers or because of other associated properties of these polymers, they have not been so-utilized. For example, it has been proposed in Japanese Patents 7591/68 and 7492/68 to form photoconductive iodinated poly(vinyl carbazoles) having at least 1 atom of iodine per molecule of the vinyl carbazole by forming either 3,6-di-iodo-9-vinylcarbazole or 3-iodo-9-vinylcarbazole from 9-chloroethylcarbazole by iodination and dehydrochlorination and subsequently polymerizing the iodinated monomer. Unfortunately, the 9-chloroethylcarbazole is expensive and difficult to obtain and the process is difficult and expensive. Therefore, the resultant iodinated polymer obtained therefrom is uneconomical and difficult to make with any significant degree of purity. These factors have severely limited the usefulness of the resultant product and have prevented their commercial utilization as photoconductive materials.
Also, it has been proposed in U.S. Pat. No. 3,507,706 to form a complex of iodine and poly(vinyl carbazole) having photovoltaic properties which prevent the final composition from being commercially useful as a photoconductive layer. In one procedure, poly(vinyl carbazole) is dissolved in a solvent to which iodine is added or has been added. Thereafter, the solvent and excess iodine are evaporated to leave poly(vinyl carbazole) complexed with iodine. This composition is undesirable in at least three major respects. First, impurities are formed in the product which degrade its photoconductive properties and which are extremely difficult to remove. In addition, the resultant product is colored so that its decreased transparency is reduced to the extent that it is not useful as a coating for white paper to form a commercially acceptable paper product or for use in forming transparencies. Finally, and most importantly, the composition is unstable and its iodine can be vaporized or reduced by reducing agents. It has also been proposed in this patent to form the iodinated polymer by exposing a film of poly(vinyl carbazole) to iodine vapors. However, this procedure produces compositions with the same disadvantages, also having limited or no commercial use.
It would be desirable to provide a transparent photoconductive iodinated polymeric composition exhibiting good dark resistivity as well as high speed electrostatic charge dissipation when exposed to light. In addition, it would be desirable to provide such a composition and a method for preparing the same which avoids the need of using expensive starting materials or the need for purifying the product. Furthermore, it would be desirable to provide such a composition which can be sensitized to visible light without significantly reducing the transparency thereof to visible light.